1. Field of the Invention
This invention relates generally to a method of assembly of a semiconductor device assembled in a flip chip configuration. More specifically, this invention relates to the application of a flux that does not require a subsequent cleaning step in the assembly process for a flip chip configuration.
2. Discussion of the Related Art
The most important objective of semiconductor packaging is to support the original design objective and intent of the integrated circuit chips. In today's technological environment, there is an ever-increasing requirement to increase the integration of circuits onto a single semiconductor chip. At the same time there is a requirement to increase the performance of the semiconductor chip, whether it is a memory chip, a microprocessor chip, a telecommunications chip or any other type of semiconductor chip. As more and more circuit functions are added to a chip, the number of interconnections also increases dramatically. An overriding factor in the increase of integration and increase of performance is the requirement to reduce the cost of the final product.
An early flip chip method of packaging semiconductors was developed in the early 1960s by IBM.RTM. as a possible replacement for the expensive, unreliable, low-productivity, and manually operated face-up wire-bonding technology. However, because high-speed automatic wire bonders for the most part met the needs of the semiconductor industry there was not an aggressive development effort expended to improve the flip chip technology methods. Flip chip technology is defined as mounting the semiconductor chip to a substrate with any kind of interconnect materials and methods such as fluxless solder bumps, tape-automated bonding (TAB), wire interconnects, conductive polymers, anisotropic conductive adhesives, metallurgy bumps, compliant bumps, and pressure contacts as long as the active chip surface is facing the substrate.
As a direct result of the higher requirements of package density, performance, and interconnection; the limitations of face-up wire bonding technology; and the growing use of multichip module technology there is a need to improve the flip chip technology and to decrease the cost of the flip chip technology at the same time. The flip chip interconnects are being used in the semiconductor industry primarily because of their high I/O density capability, small profiles, and good electrical performance. Demands on performance, reliability, and cost have resulted in the development of a variety of flip chip technologies using solder, conductive epoxy, hard metal bump (such as gold) and anisotropic conductive epoxy interconnects. Among these materials, solders have remained a preferred choice as the material forming electrical connections in flip chip assemblies.
Solder flip chip interconnect systems consist of essentially three basic elements. These include the chip, the solder bump, and the substrate. The bumps are first deposited on a wafer and reflowed. The wafer is then diced into chips. The chips are flipped over, aligned to a substrate, tacked, and reflowed. An underfill may be used to improve the reliability of the interconnects. Each of these elements and the processes used to assemble them together affect the performance and the cost of the interconnect system. Therefore, the performance and cost must be compared on the basis of the interconnect system as a whole, and not merely on any single element of the interconnect assembly.
The materials and processes involved in the manufacture of the flip chip interconnect system determine its performance. The semiconductor device or the chip may be silicon or gallium arsenide. The bond pad metallization on the wafer can be Ni--Au, Cr--Cu--Au, TiW--Cu, Ti--Cu, or TiW--Au. If the bond pads are on the substrate the selection of the bond pad metallization material depends upon the substrate material. For example, if the substrate is a ceramic material, the bond pads are Ni--Cu and if the substrate is an organic material, the bond pads are Cu. The bump material can be one of a variety of Pb-based or Pb-free solders. The substrate can be silicon, alumina, glass, or one of a variety of organic substrates. The substrate metallization can be gold or copper. Underfills are used primarily to improve reliability of flip chip interconnect systems. These underfill materials fill the gap between the chip and substrate around the solder joints, reducing the thermal stresses imposed on the solder joint.
The process step used in the manufacture of the interconnect systems can be varied and include process technologies such as plating, evaporation, wire bumping, dispensing, and printing. The reflow process may be performed in air with flux or in a controlled ambient. Flip chip bonding processes include those based on the controlled-collapse chip connection (C4) approach or those in which the geometry of the bump is controlled by the bonding equipment.
The assembly of a typical flip chip interconnect system involves two overall tasks: (1) flip chip bonding and (2) encapsulation or underfill. During flip chip bonding, the bumped die is first aligned and attached to the bond pads on the substrate using a tacky flux. (It is noted that the bumps can be formed on the substrate or on both the substrate and die and that the bond pads can be formed on the die). Then the module is heated so that the solder melts and forms a metallurgical bond with the bond pad (the reflow process). Following the flip chip bonding process the flux residues are cleaned. The solvent materials necessary to clean the flux residues are typically highly flammable and/or hazardous materials and some may be carcinogenic. Because of these characteristics of the solvent materials, the cleaning step is very expensive because it requires highly specialized equipment. The equipment may have to be explosion proof, or it may have to have special filtering systems to protect the surrounding community as well as the technicians from air pollution and/or water pollution.
Therefore, what is needed is a method of assembling a semiconductor device in a flip chip configuration without having to perform the cleaning process to remove the flux residue from the device after the reflow process is completed.